This invention relates generally to systems and methods for measuring airflow in ducts and more particularly to a system and a method for determining corrected airflow, including in aircraft turbines.
Turbines used for powering an aircraft in flight typically include, in serial communication, an air inlet duct, a fan, a compressor, a combustor, a turbine, and an exhaust duct. The combustor generates combustion gases that are channeled to the turbine where they are expanded to drive the fan and compressor. The turbine is drivingly connected to shafts coupled to the fan and compressor. Combustion is achieved by mixing ambient air with fuel and igniting the mixture. The efficiency and effectiveness of operation of the engine are dependent upon compression system pressure ratio and the air/fuel mixture and so an understanding of the airflow and the regulation of that airflow are of importance.
As indicated, the operation of the turbine engine relies on suitable ratios of air and fuel in the combustion process. It is particularly important in the field of aircraft engines that operating conditions are maintained as accurately and safely as possible. For that reason, airflow measurements are of value in assessing operating conditions of the engine. Parameters that may be assessed using airflow values under operating conditions include engine control, engine stability, engine monitoring and performance evaluations.
For a steady-state operating condition of a turbine engine, such as at a fixed throttle setting, physical airflow varies significantly as intake flow properties change. This airflow variability may be associated with changes in air density, temperature, and the like, most often associated with changes of altitude, speed and/or movement from one location to another. The variability of measured physical airflow can be reduced substantially through the process of standardizing flow data to produce a corrected airflow. The process of establishing a corrected airflow involves the manipulation of physical airflow values measured directly or indirectly to generate standardized values referenced to a common condition. For aircraft engine evaluation, the reference condition may be established by taking physical measurements through steady state operation at sea level in a stationary position.
As previously indicated, airflow values, corrected ones in particular, are of use in engine control functions, engine stability analysis, engine health monitoring, and performance evaluations. Obtaining accurate airflow data during engine design development and testing is a difficult and costly task. However, those data are necessary to verify design hypotheses and engine integrity. Moreover, they are required to create quality engine status models, engine control logic functions, and in-flight performance models.
Although accurate and reliable on-wing aircraft engine airflow measurement systems are generally considered most desirable, such systems are either not available or if available, simply not always feasible to deploy. Model-based correlation methods are therefore generally used to quantify airflow conditions. Unfortunately, these models, when relying upon physical flow data, are susceptible to multiple sources of error. As a result, modeling accuracies are limited for engines evaluated in off-line performance examinations and for real-time engine control. For this reason, additional engine stability margins must be established to ensure stall-free operation. Unfortunately, engine performance suffers when stability margins are increased. All of this leads to reduced performance for the engine user and increased development, analysis, and service costs to the manufacturer.
Physical airflow measurements are made for aircraft engines in a number of ways. The most common method is by pressure-based sensors installed within the turbine (engine) intake. Another method is by non-intrusive low-power laser-based sensors installed proximate to, but not directly in, the air stream to be measured. The pressure-based sensors are typically rake- or venturi-based flow meters that must be operated at steady-state flow conditions to be effective. They provide direct and highly accurate airflow measurement information. They also require the deployment of additional multi-probe rake systems to obtain the type of data, including air temperature, required to generate corrected airflow. Because of the expense associated with these rake or venturi systems, pressure-based sensors are unfeasible for use in mass-produced engines and can make ground-based development test structures prohibitively expensive. There are other limitations associated with the pressure-based sensors. Specifically, they cannot be used to collect meaningful flow data at high sampling rates because of probe response limitations and pneumatic lag. These are issues that do not affect laser-based sensors.
Diode laser-based airflow sensors are tuned to emit light at a frequency corresponding to a specific oxygen absorption feature within the electromagnetic spectrum. The laser beam is directed across an airflow path of interest, such as the engine inlet, and the emitted light is collected on the opposing side of the flow path. The resulting signal is proportional to the density of the air as evaluated by a signal analyzer. The laser-based sensors provide path averaging of the emitted light signal. This results in accurate outputs relatively unaffected by axisymmetric radial distortion associated with the airflow duct, regardless of laser beam orientation.
The non-intrusive laser-based flow sensor, typically diode configured, has the ability to measure simultaneously static temperature and physical airflow. Moreover, it is relatively easier to install for ground-test applications and for on-wing measurements in real-time conditions with little loss of accuracy. It therefore eliminates the need for complex airflow models designed to correlate test parameters with real-time conditions. The resulting data may be used directly in an engine control processing system.
Existing pressure-based airflow sensors are subject to providing unreliable physical airflow information under many test and real-time operating conditions. Corresponding correction of the physical airflow parameters obtained from such sensors requires complex modeling and supplemental multi-probe rake arrangements. Suitably accurate pressure-based sensors are therefore often too expensive to place in production or use in development testing. Laser-based sensors offer a desirable technique for feasibly obtaining reliable physical airflow information. However, what is needed is a feasible system and method for obtaining corrected airflow using laser-based sensors.
The above-mentioned need is met by the present invention, which provides a system and methodology for obtaining reliable corrected gas flow, and for aircraft engines in particular, airflow, values. The method includes generating a corrected gas flow value from a physical gas flow value for a gas moving through a duct of a gas turbine engine. This is achieved by applying a laser-based sensor to an interior of the duct and measuring the physical gas flow value WPhysical of the gas with the laser-based sensor. The direct measurements provide for a non-intrusive technique to obtain data that is then used in a processing unit, such as any sort of computing system programmed to calculate the corrected gas flow value WCorr.
The system to enable the described methodology includes the laser-based sensor system placed non-intrusively in the duct that is the gas flow passageway. The system also includes the processing unit coupled to the sensor system. In particular, the system of the present invention measures the physical gas flow value WPhysical of the gas flowing through the duct and calculates the corrected gas flow value WCorr from WPhysical. The laser-based sensor is affixed to the interior of the duct and measures characteristics associated with physical parameters of the gas moving in the duct. The processor is coupled to the laser-based sensor for controlling operation of the sensor, receiving signal information from the sensor, and calculating WPhysical and WCorr. The sensor system includes two or more pairs of photoemitters/photodetectors aligned to one another in a way that generates gas flow velocity using a Doppler shift. The beams created by the pairs are also analyzed to generate absorption features that are used to generate gas density information. The processor may also be programmed to measure static temperature in the duct using a second laser beam and based on relative variations of absorption features between the two beams.